Kusasalethu is situated in the West Wits Basin and mines the Ventersdorp Contact Reef as its main ore body. The Ventersdorp Contact Reef rests uncomformably on the quartzites of the Witwatersrand Supergroup.

These Witwatersrand quartzites belong to the Mondeor Formation in the western part of the lease area and the Elsburgs Quartzite Formation in the eastern part of the lease area. The unconformity angle becomes more perceptible towards the east. The Ventersdorp Contact Reef has an average dip of 25 degrees to the southeast and an average strike of N72 degrees east.

The Ventersdorp Contact Reef is generally a clast-supported conglomerate of small sub angular to sub-rounded milky and smoky (60:40 respectively) quartz pebbles. The matrix is dark grey and medium-grained and comprises mostly quartzite, separating the two units as internal quartzites. It is mineralised by some pyrrhotite, chalcopyrite and, in rare instances, by some carbon flyspecks. Sometimes there are changes to the reef appearance in the form of thickness and, to some degree, elimination. These changes are brought about by either erosion (lava erosion channels – lava appearing at different elevations, with resultant undulations of the reef), or flat faulting (as evidenced by the presence of mylonite at the top contact of the reef).

The Elsburgs conglomerates are found on the western side of Kusasalethu, forming the footwall to the Ventersdorp Contact Reef. The Elsburgs are part of the Turffontein Supergroup. It is a predominantly polymictic matrix supported conglomerate of well-packed and moderately sorted, sub-rounded smoky (80%), black/grey (15%) quartz pebbles, chert (3%) and some elongated shale pebbles (2%). The matrix is pale yellow to light green and medium grained, also, pyritic in places.

The Ventersdorp Contact Reef is overlain by the Ventersdorp Lava. The lava belongs to the Ventersdorp Supergroup. It is light to midgrey in colour and fine crystalline, seldom containing phenocrysts. In places it is amygdaloidal with quartz and pyrite mineralisation. Flow structures are also present at the base of the lava. It breaks into very angular fragments due to weak jointing and the flow banding – it would appear to be andesitic in composition.

The mine comprises twin vertical and twin sub vertical shaft systems and uses conventional mining methods in a sequential grid layout. Mining is conducted to a depth of 3 388m, making it Harmony’s deepest mine.

The 10m-diameter rock/ventilation shaft was sunk to 2 195m and the man/material shaft to 2 127m. By June 1984, a 10m-diameter subvertical rock/service shaft was completed to a depth of 3 048m and a 7m-diameter sub-vertical ventilation shaft to a depth of 3 048m. Both of these shafts were deepened to a final depth below surface of 3 318m and 3 388m respectively as part of the deepening project to extract the higher-grade pay shoot towards the west of the mine. In December 2014, a decision was taken to suspend operations in the old portion of Kusasalethu and to restructure the mine. Subsequently mining above 98 level ceased.

Mining is by means of sequential grid mining with regional dip stabilising pillars, backfill and pre-conditioning to offset the effects of mining at this depth. Mining is conducted over five levels from 98 level to 113 level. Large geological structures are stabilised by means of clamping pillars.

Ore mined is processed on site at the Kusasalethu gold plant. Gold is extracted by means of milling, cyanide leaching, carbonin- pulp concentration and electrowinning to absorb the carbon to produce doré.

Milling
The milling section consists of three ROM mills with design capacity of 140 tons per hour (t/h). The three parallel and independent milling lines, each having a conveyor underneath each silo, get material delivered onto the conveyor via a Langlaagte chute. Steel balls are used as grinding media, supplemented by wide range particle size of mill feed. The current feed rate is 75t/h. The mill feed and discharge pumps have variable drives to enable control of optimal efficiency for grinding and classification. There is two stage classification for each mill with each primary cyclone overflow combined into a common tank, the secondary cyclone feed tank, for second stage classification by secondary cyclone. The overflow of the secondary cyclone is further screened on a linear screen for the removal of coarse woodchips and tramp steel. The material passing through the linear screen is normally at a density of 1.1 -1.2 t/m3 and a grind of 80% -75 micron and allowed to gravitate to the thickeners for dewatering.

Thickening
There are three thickeners, two of which are always online, fed from a distribution box of the thickeners feed launder, with the one thickener available as a standby in case of emergency. Calcium oxide (lime) is added at the thickeners for maintaining a level of alkalinity with a pH of 10.4 to prevent the generation of poisonous HCN (cyanide) gas at the leach section. Flocculant is added to aid with settling of solids, pulping the slime to densities of 1.5 to 1.6 t/m3. Each thickener is equipped with variable speed drives to control leach feed densities which are linked to the installed densitometers that speeds or slows down the pump, based on the underflow densities. The overflow water of the thickeners gravitates to the two mill return tanks to be reused in the milling process.

Leach
The leach circuit consist of 12 tanks, each with a height of 20m and diameter of 9.0m Cyanide is automatically added to the first tank at a concentration of 240ppm, for dissolution of gold. Compressed air is used to agitate the slurry, suspending solids in pulp and raising dissolved oxygen to about 10ppm for optimal leach efficiencies. Retention time in each tank is 3.5 hours, totalling a 35 hour leach period for the 9 tanks online. Leach slurry flows from first tank through to the eleventh tank, while consuming 140ppm of cyanide in the process.

Carbon in pulp
The carbon in pulp (CIP) circuit is a carousel system design with eight tanks of 3.6m diameters and 9m height each. A cylindrical screen with 0.630mm apertures is used in every tank to retain activated carbon added at concentration of 50g/l. The first tank loads up to 10Kg/t by adsorbing the dissolved gold while allowable dissolved losses at the last tank on line is controlled at 0.01g/t. The loaded head tank is dropped and bagged to be transported to Evander Kinross plant for further refining process.